1. Field of the Invention
The present invention relates to a multilayer phase change information recording medium for use in recording and playing back information with light such as a laser beam and to a recording and playback method for the medium.
2. Discussion of the Background
Generally, phase change optical discs (phase change information recording media) such as CD-RWs contain a plastic substrate, a recording layer and a reflective layer. The recording layer is made of a phase change material and is formed on the plastic substrate. The reflective layer is formed on the recording layer in order to improve optical absorptivity of the recording layer. The reflective layer also has a heat diffusion effect. Information is recorded and played back with a laser beam.
When a laser beam irradiates a phase change material, the phase change material phase-changes between a crystalline state and an amorphous state. An amorphous state is achieved upon rapid heating followed by rapid cooling and a crystalline state is achieved upon rapid heating followed by gradual cooling. Phase change information recording media utilizes this characteristic to record and play back information.
Further, in order to prevent oxidation, evaporation and transformation of the recording layer due to the heat of a laser beam, a lower protective layer (also referred to as lower dielectric layer) and an upper protective layer (also referred to as upper dielectric layer) are provided between the substrate and the recording layer and between the recording layer and the reflective layer, respectively.
Furthermore, the optical characteristics of a recording medium can be adjusted by the thickness of the protective layers. In addition, the lower protective layer has also a function of preventing the substrate from softening due to the heat generated at recording.
Recently, the amount of information dealt with by computers has been increasing. Therefore the signal recording capacity of optical discs such as DVD-RAMs and DVD+RWs and the density of the signal information has increased.
Currently, the recording capacities of CDs and DVDs are about 650 MB and about 4.7 GB, respectively. From now on, an increasing demand will be expected for high recording capacity and high density.
In attempting to prepare a medium having a high recording capacity and density using such phase change information recording media, there is a proposal in which the wavelength of a laser beam used is made to be shifted to a blue ray region. There is another proposal in which the spot size of a laser beam irradiating an optical medium is made to be small by using a high NA objective lens for a pickup for recording and playback.
In attempting to obtain a high recording capacity by improving information recording media itself, for example, Japanese Patent No. 2702905 and published unexamined Japanese Patent Application Nos. 2000-215516, 2000-222777 and 2001-243566 have disclosed double layer phase change information recording media wherein two information layers, each of which contains at least a recording layer and a reflective layer, are overlaid on one side of a substrate. The two information layers are bonded to each other with an ultraviolet curing resin or the like.
This bonding portion between the information layers is a separation layer (referred to as an intermediate layer in the present invention). This separation layer has a function of optically separating the two information layers and is made of materials which absorb light as little as possible in order that the laser beam for use in recording and playback reaches the inner information layer as much as possible.
The first information layer which includes a first protective layer and a second protective layer is characteristic to all of these double layer phase change information recording media as in the case of a single layer phase change information recording medium.
This type of double layer phase change information recording media was announced in, for example, academic conference (e.g., ODS2001 Technical Digest P22-24 (P24 FIG. 5)) but there are still many issues to be solved.
A laser beam should sufficiently transmit through the first information layer which receives the laser beam first in order to record and play back information in the recording layer of the second information layer. Thinkable measures to this issue are that the reflective layer included in the first information layer is removed or made to be extremely thin, or the recording layer included in the first information layer is made to be extremely thin.
Information is recorded in a phase change information recording medium by marks formed when a crystalline state of the recording layer is phase changed to an amorphous state upon application of a laser beam to the phase change material and subsequent rapid cooling.
Therefore, when no reflective layer or an extremely thin layer, for example, with a thickness of approximately 10 nm, is provided, the heat diffusion effect decreases, resulting in difficulty in forming amorphous marks.
Especially, an Sb—Te eutectic crystal recording material, which is one of the typical materials for use in phase change information recording media such as CD-RWs, is excellent in erase ratio compared with a Ge—Sb—Te compound containing recording material. In addition, such an Sb—Te eutectic crystal recording material is so sensitive that an amorphous recording mark can have a clear contour.
However, an Sb—Te eutectic crystal recording material has relatively fast crystallization speed compared to a Ge—Sb—Te compound containing recording material. Therefore, rapid cooling needs to be performed in a short time to achieve an amorphous state and a structure suitable for such rapid cooling is required. Therefore, forming marks becomes difficult using this material when a reflective layer is extremely thin.
In attempting to clear the problem mentioned above that occurs in the first information layer having a thin reflective layer, Japanese Patent Application Nos. 8-50739 and 2000-222777 have disclosed a single layer phase change information medium and a double layer phase change information medium, respectively. The method of the media is that a heat diffusion layer consisting of compounds having a relatively high thermal conductivity and a relatively low light absorptivity such as nitrides and carbides is provided on the reflective layer to improve the heat diffusion function of the reflective layer. This method is thought to be effective to clear the defects occurring when a thin layer is used.
However, these materials such as nitrides and carbides have a large stress so that the heat diffusion layer tends to crack. As a result, there is a problem in that optical discs having such a heat diffusion layer cannot obtain sufficient overwriting characteristics.
Because of these reasons, a need exists for a multilayer phase change information recording medium which has excellent overwrite characteristics even when the reflective layer contained in the information layer is thin.